qdatetime-utc: Rework test for even better performance and more usecases, add proper Qt6/undeprecated Qt5 support

The documented results can be verified using a simple benchmark test

        QBENCHMARK {
            QDateTime::currentDateTime().toMSecsSinceEpoch();
        }
        QBENCHMARK {
            QDateTime::currentDateTimeUtc().toMSecsSinceEpoch();
        }
        QBENCHMARK {
            QDateTime::currentMSecsSinceEpoch();
        }